An aria (Italian for air; plural: arie or arias in common usage) in music was originally any expressive melody, usually, but not always, performed by a singer. It is now used almost exclusively to describe a self contained piece for one voice usually with orchestral accompaniment, especially as part of an opera.
In the 17th century, the aria was written in ternary form (ABA); these arias were known as arie col da capo). The aria later "invaded" the opera repertoire with its many sub-species (Aria cantabile, Aria agitata, Aria di bravura, and so on). In the 18th century, operas became a sequence of arias, reducing the space left for recitative.